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On the Origin of Species, 6th Edition by Charles Darwin

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Hence we ought not to expect at the present time to meet with numerous
transitional varieties in each region, though they must have existed there,
and may be embedded there in a fossil condition. But in the intermediate
region, having intermediate conditions of life, why do we not now find
closely-linking intermediate varieties? This difficulty for a long time
quite confounded me. But I think it can be in large part explained.

In the first place we should be extremely cautious in inferring, because an
area is now continuous, that it has been continuous during a long period.
Geology would lead us to believe that most continents have been broken up
into islands even during the later tertiary periods; and in such islands
distinct species might have been separately formed without the possibility
of intermediate varieties existing in the intermediate zones. By changes
in the form of the land and of climate, marine areas now continuous must
often have existed within recent times in a far less continuous and uniform
condition than at present. But I will pass over this way of escaping from
the difficulty; for I believe that many perfectly defined species have been
formed on strictly continuous areas; though I do not doubt that the
formerly broken condition of areas now continuous, has played an important
part in the formation of new species, more especially with freely-crossing
and wandering animals.

In looking at species as they are now distributed over a wide area, we
generally find them tolerably numerous over a large territory, then
becoming somewhat abruptly rarer and rarer on the confines, and finally
disappearing. Hence the neutral territory between two representative
species is generally narrow in comparison with the territory proper to
each. We see the same fact in ascending mountains, and sometimes it is
quite remarkable how abruptly, as Alph. De Candolle has observed, a common
alpine species disappears. The same fact has been noticed by E. Forbes in
sounding the depths of the sea with the dredge. To those who look at
climate and the physical conditions of life as the all-important elements
of distribution, these facts ought to cause surprise, as climate and height
or depth graduate away insensibly. But when we bear in mind that almost
every species, even in its metropolis, would increase immensely in numbers,
were it not for other competing species; that nearly all either prey on or
serve as prey for others; in short, that each organic being is either
directly or indirectly related in the most important manner to other
organic beings--we see that the range of the inhabitants of any country by
no means exclusively depends on insensibly changing physical conditions,
but in large part on the presence of other species, on which it lives, or
by which it is destroyed, or with which it comes into competition; and as
these species are already defined objects, not blending one into another by
insensible gradations, the range of any one species, depending as it does
on the range of others, will tend to be sharply defined. Moreover, each
species on the confines of its range, where it exists in lessened numbers,
will, during fluctuations in the number of its enemies or of its prey, or
in the nature of the seasons, be extremely liable to utter extermination;
and thus its geographical range will come to be still more sharply defined.

As allied or representative species, when inhabiting a continuous area, are
generally distributed in such a manner that each has a wide range, with a
comparatively narrow neutral territory between them, in which they become
rather suddenly rarer and rarer; then, as varieties do not essentially
differ from species, the same rule will probably apply to both; and if we
take a varying species inhabiting a very large area, we shall have to adapt
two varieties to two large areas, and a third variety to a narrow
intermediate zone. The intermediate variety, consequently, will exist in
lesser numbers from inhabiting a narrow and lesser area; and practically,
as far as I can make out, this rule holds good with varieties in a state of
nature. I have met with striking instances of the rule in the case of
varieties intermediate between well-marked varieties in the genus Balanus.
And it would appear from information given me by Mr. Watson, Dr. Asa Gray,
and Mr. Wollaston, that generally, when varieties intermediate between two
other forms occur, they are much rarer numerically than the forms which
they connect. Now, if we may trust these facts and inferences, and
conclude that varieties linking two other varieties together generally have
existed in lesser numbers than the forms which they connect, then we can
understand why intermediate varieties should not endure for very long
periods: why, as a general rule, they should be exterminated and
disappear, sooner than the forms which they originally linked together.

For any form existing in lesser numbers would, as already remarked, run a
greater chance of being exterminated than one existing in large numbers;
and in this particular case the intermediate form would be eminently liable
to the inroads of closely allied forms existing on both sides of it. But
it is a far more important consideration, that during the process of
further modification, by which two varieties are supposed to be converted
and perfected into two distinct species, the two which exist in larger
numbers, from inhabiting larger areas, will have a great advantage over the
intermediate variety, which exists in smaller numbers in a narrow and
intermediate zone. For forms existing in larger numbers will have a better
chance, within any given period, of presenting further favourable
variations for natural selection to seize on, than will the rarer forms
which exist in lesser numbers. Hence, the more common forms, in the race
for life, will tend to beat and supplant the less common forms, for these
will be more slowly modified and improved. It is the same principle which,
as I believe, accounts for the common species in each country, as shown in
the second chapter, presenting on an average a greater number of
well-marked varieties than do the rarer species. I may illustrate what I
mean by supposing three varieties of sheep to be kept, one adapted to an
extensive mountainous region; a second to a comparatively narrow, hilly
tract; and a third to the wide plains at the base; and that the inhabitants
are all trying with equal steadiness and skill to improve their stocks by
selection; the chances in this case will be strongly in favour of the great
holders on the mountains or on the plains improving their breeds more
quickly than the small holders on the intermediate narrow, hilly tract; and
consequently the improved mountain or plain breed will soon take the place
of the less improved hill breed; and thus the two breeds, which originally
existed in greater numbers, will come into close contact with each other,
without the interposition of the supplanted, intermediate hill variety.

To sum up, I believe that species come to be tolerably well-defined
objects, and do not at any one period present an inextricable chaos of
varying and intermediate links: first, because new varieties are very
slowly formed, for variation is a slow process, and natural selection can
do nothing until favourable individual differences or variations occur, and
until a place in the natural polity of the country can be better filled by
some modification of some one or more of its inhabitants. And such new
places will depend on slow changes of climate, or on the occasional
immigration of new inhabitants, and, probably, in a still more important
degree, on some of the old inhabitants becoming slowly modified, with the
new forms thus produced and the old ones acting and reacting on each other.
So that, in any one region and at any one time, we ought to see only a few
species presenting slight modifications of structure in some degree
permanent; and this assuredly we do see.

Secondly, areas now continuous must often have existed within the recent
period as isolated portions, in which many forms, more especially among the
classes which unite for each birth and wander much, may have separately
been rendered sufficiently distinct to rank as representative species. In
this case, intermediate varieties between the several representative
species and their common parent, must formerly have existed within each
isolated portion of the land, but these links during the process of natural
selection will have been supplanted and exterminated, so that they will no
longer be found in a living state.

Thirdly, when two or more varieties have been formed in different portions
of a strictly continuous area, intermediate varieties will, it is probable,
at first have been formed in the intermediate zones, but they will
generally have had a short duration. For these intermediate varieties
will, from reasons already assigned (namely from what we know of the actual
distribution of closely allied or representative species, and likewise of
acknowledged varieties), exist in the intermediate zones in lesser numbers
than the varieties which they tend to connect. From this cause alone the
intermediate varieties will be liable to accidental extermination; and
during the process of further modification through natural selection, they
will almost certainly be beaten and supplanted by the forms which they
connect; for these, from existing in greater numbers will, in the
aggregate, present more varieties, and thus be further improved through
natural selection and gain further advantages.

Lastly, looking not to any one time, but at all time, if my theory be true,
numberless intermediate varieties, linking closely together all the species
of the same group, must assuredly have existed; but the very process of
natural selection constantly tends, as has been so often remarked, to
exterminate the parent forms and the intermediate links. Consequently
evidence of their former existence could be found only among fossil remains,
which are preserved, as we shall attempt to show in a future chapter, in an
extremely imperfect and intermittent record.


It has been asked by the opponents of such views as I hold, how, for
instance, could a land carnivorous animal have been converted into one with
aquatic habits; for how could the animal in its transitional state have
subsisted? It would be easy to show that there now exist carnivorous
animals presenting close intermediate grades from strictly terrestrial to
aquatic habits; and as each exists by a struggle for life, it is clear that
each must be well adapted to its place in nature. Look at the Mustela
vison of North America, which has webbed feet, and which resembles an otter
in its fur, short legs, and form of tail; during summer this animal dives
for and preys on fish, but during the long winter it leaves the frozen
waters, and preys, like other polecats on mice and land animals. If a
different case had been taken, and it had been asked how an insectivorous
quadruped could possibly have been converted into a flying bat, the
question would have been far more difficult to answer. Yet I think such
difficulties have little weight.

Here, as on other occasions, I lie under a heavy disadvantage, for, out of
the many striking cases which I have collected, I can give only one or two
instances of transitional habits and structures in allied species; and of
diversified habits, either constant or occasional, in the same species.
And it seems to me that nothing less than a long list of such cases is
sufficient to lessen the difficulty in any particular case like that of the

Look at the family of squirrels; here we have the finest gradation from
animals with their tails only slightly flattened, and from others, as Sir
J. Richardson has remarked, with the posterior part of their bodies rather
wide and with the skin on their flanks rather full, to the so-called flying
squirrels; and flying squirrels have their limbs and even the base of the
tail united by a broad expanse of skin, which serves as a parachute and
allows them to glide through the air to an astonishing distance from tree
to tree. We cannot doubt that each structure is of use to each kind of
squirrel in its own country, by enabling it to escape birds or beasts of
prey, or to collect food more quickly, or, as there is reason to believe,
to lessen the danger from occasional falls. But it does not follow from
this fact that the structure of each squirrel is the best that it is
possible to conceive under all possible conditions. Let the climate and
vegetation change, let other competing rodents or new beasts of prey
immigrate, or old ones become modified, and all analogy would lead us to
believe that some, at least, of the squirrels would decrease in numbers or
become exterminated, unless they also become modified and improved in
structure in a corresponding manner. Therefore, I can see no difficulty,
more especially under changing conditions of life, in the continued
preservation of individuals with fuller and fuller flank-membranes, each
modification being useful, each being propagated, until, by the accumulated
effects of this process of natural selection, a perfect so-called flying
squirrel was produced.

Now look at the Galeopithecus or so-called flying lemur, which was formerly
ranked among bats, but is now believed to belong to the Insectivora. An
extremely wide flank-membrane stretches from the corners of the jaw to the
tail, and includes the limbs with the elongated fingers. This flank-
membrane is furnished with an extensor muscle. Although no graduated links
of structure, fitted for gliding through the air, now connect the
Galeopithecus with the other Insectivora, yet there is no difficulty in
supposing that such links formerly existed, and that each was developed in
the same manner as with the less perfectly gliding squirrels; each grade of
structure having been useful to its possessor. Nor can I see any
insuperable difficulty in further believing it possible that the
membrane-connected fingers and fore-arm of the Galeopithecus might have
been greatly lengthened by natural selection; and this, as far as the
organs of flight are concerned, would have converted the animal into a bat.
In certain bats in which the wing-membrane extends from the top of the
shoulder to the tail and includes the hind-legs, we perhaps see traces of
an apparatus originally fitted for gliding through the air rather than for

If about a dozen genera of birds were to become extinct, who would have
ventured to surmise that birds might have existed which used their wings
solely as flappers, like the logger headed duck (Micropterus of Eyton); as
fins in the water and as front legs on the land, like the penguin; as
sails, like the ostrich; and functionally for no purpose, like the apteryx?
Yet the structure of each of these birds is good for it, under the
conditions of life to which it is exposed, for each has to live by a
struggle: but it is not necessarily the best possible under all possible
conditions. It must not be inferred from these remarks that any of the
grades of wing-structure here alluded to, which perhaps may all be the
result of disuse, indicate the steps by which birds actually acquired their
perfect power of flight; but they serve to show what diversified means of
transition are at least possible.

Seeing that a few members of such water-breathing classes as the Crustacea
and Mollusca are adapted to live on the land; and seeing that we have
flying birds and mammals, flying insects of the most diversified types, and
formerly had flying reptiles, it is conceivable that flying-fish, which now
glide far through the air, slightly rising and turning by the aid of their
fluttering fins, might have been modified into perfectly winged animals.
If this had been effected, who would have ever imagined that in an early
transitional state they had been inhabitants of the open ocean, and had
used their incipient organs of flight exclusively, so far as we know, to
escape being devoured by other fish?

When we see any structure highly perfected for any particular habit, as the
wings of a bird for flight, we should bear in mind that animals displaying
early transitional grades of the structure will seldom have survived to the
present day, for they will have been supplanted by their successors, which
were gradually rendered more perfect through natural selection.
Furthermore, we may conclude that transitional states between structures
fitted for very different habits of life will rarely have been developed at
an early period in great numbers and under many subordinate forms. Thus,
to return to our imaginary illustration of the flying-fish, it does not
seem probable that fishes capable of true flight would have been developed
under many subordinate forms, for taking prey of many kinds in many ways,
on the land and in the water, until their organs of flight had come to a
high stage of perfection, so as to have given them a decided advantage over
other animals in the battle for life. Hence the chance of discovering
species with transitional grades of structure in a fossil condition will
always be less, from their having existed in lesser numbers, than in the
case of species with fully developed structures.

I will now give two or three instances, both of diversified and of changed
habits, in the individuals of the same species. In either case it would be
easy for natural selection to adapt the structure of the animal to its
changed habits, or exclusively to one of its several habits. It is,
however, difficult to decide and immaterial for us, whether habits
generally change first and structure afterwards; or whether slight
modifications of structure lead to changed habits; both probably often
occurring almost simultaneously. Of cases of changed habits it will
suffice merely to allude to that of the many British insects which now feed
on exotic plants, or exclusively on artificial substances. Of diversified
habits innumerable instances could be given: I have often watched a tyrant
flycatcher (Saurophagus sulphuratus) in South America, hovering over one
spot and then proceeding to another, like a kestrel, and at other times
standing stationary on the margin of water, and then dashing into it like a
kingfisher at a fish. In our own country the larger titmouse (Parus major)
may be seen climbing branches, almost like a creeper; it sometimes, like a
shrike, kills small birds by blows on the head; and I have many times seen
and heard it hammering the seeds of the yew on a branch, and thus breaking
them like a nuthatch. In North America the black bear was seen by Hearne
swimming for hours with widely open mouth, thus catching, almost like a
whale, insects in the water.

As we sometimes see individuals following habits different from those
proper to their species and to the other species of the same genus, we
might expect that such individuals would occasionally give rise to new
species, having anomalous habits, and with their structure either slightly
or considerably modified from that of their type. And such instances occur
in nature. Can a more striking instance of adaptation be given than that
of a woodpecker for climbing trees and seizing insects in the chinks of the
bark? Yet in North America there are woodpeckers which feed largely on
fruit, and others with elongated wings which chase insects on the wing. On
the plains of La Plata, where hardly a tree grows, there is a woodpecker
(Colaptes campestris) which has two toes before and two behind, a long-
pointed tongue, pointed tail-feathers, sufficiently stiff to support the
bird in a vertical position on a post, but not so stiff as in the typical
wood-peckers, and a straight, strong beak. The beak, however, is not so
straight or so strong as in the typical woodpeckers but it is strong enough
to bore into wood. Hence this Colaptes, in all the essential parts of its
structure, is a woodpecker. Even in such trifling characters as the
colouring, the harsh tone of the voice, and undulatory flight, its close
blood-relationship to our common woodpecker is plainly declared; yet, as I
can assert, not only from my own observations, but from those of the
accurate Azara, in certain large districts it does not climb trees, and it
makes its nest in holes in banks! In certain other districts, however,
this same woodpecker, as Mr. Hudson states, frequents trees, and bores
holes in the trunk for its nest. I may mention as another illustration of
the varied habits of this genus, that a Mexican Colaptes has been described
by De Saussure as boring holes into hard wood in order to lay up a store of

Petrels are the most aerial and oceanic of birds, but, in the quiet sounds
of Tierra del Fuego, the Puffinuria berardi, in its general habits, in its
astonishing power of diving, in its manner of swimming and of flying when
made to take flight, would be mistaken by any one for an auk or a grebe;
nevertheless, it is essentially a petrel, but with many parts of its
organisation profoundly modified in relation to its new habits of life;
whereas the woodpecker of La Plata has had its structure only slightly
modified. In the case of the water-ouzel, the acutest observer, by
examining its dead body, would never have suspected its sub-aquatic habits;
yet this bird, which is allied to the thrush family, subsists by
diving,--using its wings under water and grasping stones with its feet.
All the members of the great order of Hymenopterous insects are
terrestrial, excepting the genus Proctotrupes, which Sir John Lubbock has
discovered to be aquatic in its habits; it often enters the water and dives
about by the use not of its legs but of its wings, and remains as long as
four hours beneath the surface; yet it exhibits no modification in
structure in accordance with its abnormal habits.

He who believes that each being has been created as we now see it, must
occasionally have felt surprise when he has met with an animal having
habits and structure not in agreement. What can be plainer than that the
webbed feet of ducks and geese are formed for swimming? Yet there are
upland geese with webbed feet which rarely go near the water; and no one
except Audubon, has seen the frigate-bird, which has all its four toes
webbed, alight on the surface of the ocean. On the other hand, grebes and
coots are eminently aquatic, although their toes are only bordered by
membrane. What seems plainer than that the long toes, not furnished with
membrane, of the Grallatores, are formed for walking over swamps and
floating plants. The water-hen and landrail are members of this order, yet
the first is nearly as aquatic as the coot, and the second is nearly as
terrestrial as the quail or partridge. In such cases, and many others
could be given, habits have changed without a corresponding change of
structure. The webbed feet of the upland goose may be said to have become
almost rudimentary in function, though not in structure. In the
frigate-bird, the deeply scooped membrane between the toes shows that
structure has begun to change.

He who believes in separate and innumerable acts of creation may say, that
in these cases it has pleased the Creator to cause a being of one type to
take the place of one belonging to another type; but this seems to me only
restating the fact in dignified language. He who believes in the struggle
for existence and in the principle of natural selection, will acknowledge
that every organic being is constantly endeavouring to increase in numbers;
and that if any one being varies ever so little, either in habits or
structure, and thus gains an advantage over some other inhabitant of the
same country, it will seize on the place of that inhabitant, however
different that may be from its own place. Hence it will cause him no
surprise that there should be geese and frigate-birds with webbed feet,
living on the dry land and rarely alighting on the water, that there should
be long-toed corncrakes, living in meadows instead of in swamps; that there
should be woodpeckers where hardly a tree grows; that there should be
diving thrushes and diving Hymenoptera, and petrels with the habits of


To suppose that the eye with all its inimitable contrivances for adjusting
the focus to different distances, for admitting different amounts of light,
and for the correction of spherical and chromatic aberration, could have
been formed by natural selection, seems, I freely confess, absurd in the
highest degree. When it was first said that the sun stood still and the
world turned round, the common sense of mankind declared the doctrine
false; but the old saying of Vox populi, vox Dei, as every philosopher
knows, cannot be trusted in science. Reason tells me, that if numerous
gradations from a simple and imperfect eye to one complex and perfect can
be shown to exist, each grade being useful to its possessor, as is
certainly the case; if further, the eye ever varies and the variations be
inherited, as is likewise certainly the case; and if such variations should
be useful to any animal under changing conditions of life, then the
difficulty of believing that a perfect and complex eye could be formed by
natural selection, though insuperable by our imagination, should not be
considered as subversive of the theory. How a nerve comes to be sensitive
to light, hardly concerns us more than how life itself originated; but I
may remark that, as some of the lowest organisms in which nerves cannot be
detected, are capable of perceiving light, it does not seem impossible that
certain sensitive elements in their sarcode should become aggregated and
developed into nerves, endowed with this special sensibility.

In searching for the gradations through which an organ in any species has
been perfected, we ought to look exclusively to its lineal progenitors; but
this is scarcely ever possible, and we are forced to look to other species
and genera of the same group, that is to the collateral descendants from
the same parent-form, in order to see what gradations are possible, and for
the chance of some gradations having been transmitted in an unaltered or
little altered condition. But the state of the same organ in distinct
classes may incidentally throw light on the steps by which it has been

The simplest organ which can be called an eye consists of an optic nerve,
surrounded by pigment-cells and covered by translucent skin, but without
any lens or other refractive body. We may, however, according to M.
Jourdain, descend even a step lower and find aggregates of pigment-cells,
apparently serving as organs of vision, without any nerves, and resting
merely on sarcodic tissue. Eyes of the above simple nature are not capable
of distinct vision, and serve only to distinguish light from darkness. In
certain star-fishes, small depressions in the layer of pigment which
surrounds the nerve are filled, as described by the author just quoted,
with transparent gelatinous matter, projecting with a convex surface, like
the cornea in the higher animals. He suggests that this serves not to form
an image, but only to concentrate the luminous rays and render their
perception more easy. In this concentration of the rays we gain the first
and by far the most important step towards the formation of a true,
picture-forming eye; for we have only to place the naked extremity of the
optic nerve, which in some of the lower animals lies deeply buried in the
body, and in some near the surface, at the right distance from the
concentrating apparatus, and an image will be formed on it.

In the great class of the Articulata, we may start from an optic nerve
simply coated with pigment, the latter sometimes forming a sort of pupil,
but destitute of lens or other optical contrivance. With insects it is now
known that the numerous facets on the cornea of their great compound eyes
form true lenses, and that the cones include curiously modified nervous
filaments. But these organs in the Articulata are so much diversified that
Muller formerly made three main classes with seven subdivisions, besides a
fourth main class of aggregated simple eyes.

When we reflect on these facts, here given much too briefly, with respect
to the wide, diversified, and graduated range of structure in the eyes of
the lower animals; and when we bear in mind how small the number of all
living forms must be in comparison with those which have become extinct,
the difficulty ceases to be very great in believing that natural selection
may have converted the simple apparatus of an optic nerve, coated with
pigment and invested by transparent membrane, into an optical instrument as
perfect as is possessed by any member of the Articulata class.

He who will go thus far, ought not to hesitate to go one step further, if
he finds on finishing this volume that large bodies of facts, otherwise
inexplicable, can be explained by the theory of modification through
natural selection; he ought to admit that a structure even as perfect as an
eagle's eye might thus be formed, although in this case he does not know
the transitional states. It has been objected that in order to modify the
eye and still preserve it as a perfect instrument, many changes would have
to be effected simultaneously, which, it is assumed, could not be done
through natural selection; but as I have attempted to show in my work on
the variation of domestic animals, it is not necessary to suppose that the
modifications were all simultaneous, if they were extremely slight and
gradual. Different kinds of modification would, also, serve for the same
general purpose: as Mr. Wallace has remarked, "If a lens has too short or
too long a focus, it may be amended either by an alteration of curvature,
or an alteration of density; if the curvature be irregular, and the rays do
not converge to a point, then any increased regularity of curvature will be
an improvement. So the contraction of the iris and the muscular movements
of the eye are neither of them essential to vision, but only improvements
which might have been added and perfected at any stage of the construction
of the instrument." Within the highest division of the animal kingdom,
namely, the Vertebrata, we can start from an eye so simple, that it
consists, as in the lancelet, of a little sack of transparent skin,
furnished with a nerve and lined with pigment, but destitute of any other
apparatus. In fishes and reptiles, as Owen has remarked, "The range of
gradation of dioptric structures is very great." It is a significant fact
that even in man, according to the high authority of Virchow, the beautiful
crystalline lens is formed in the embryo by an accumulation of epidermic
cells, lying in a sack-like fold of the skin; and the vitreous body is
formed from embryonic subcutaneous tissue. To arrive, however, at a just
conclusion regarding the formation of the eye, with all its marvellous yet
not absolutely perfect characters, it is indispensable that the reason
should conquer the imagination; but I have felt the difficulty far to
keenly to be surprised at others hesitating to extend the principle of
natural selection to so startling a length.

It is scarcely possible to avoid comparing the eye with a telescope. We
know that this instrument has been perfected by the long-continued efforts
of the highest human intellects; and we naturally infer that the eye has
been formed by a somewhat analogous process. But may not this inference be
presumptuous? Have we any right to assume that the Creator works by
intellectual powers like those of man? If we must compare the eye to an
optical instrument, we ought in imagination to take a thick layer of
transparent tissue, with spaces filled with fluid, and with a nerve
sensitive to light beneath, and then suppose every part of this layer to be
continually changing slowly in density, so as to separate into layers of
different densities and thicknesses, placed at different distances from
each other, and with the surfaces of each layer slowly changing in form.
Further we must suppose that there is a power, represented by natural
selection or the survival of the fittest, always intently watching each
slight alteration in the transparent layers; and carefully preserving each
which, under varied circumstances, in any way or degree, tends to produce a
distincter image. We must suppose each new state of the instrument to be
multiplied by the million; each to be preserved until a better is produced,
and then the old ones to be all destroyed. In living bodies, variation
will cause the slight alteration, generation will multiply them almost
infinitely, and natural selection will pick out with unerring skill each
improvement. Let this process go on for millions of years; and during each
year on millions of individuals of many kinds; and may we not believe that
a living optical instrument might thus be formed as superior to one of
glass, as the works of the Creator are to those of man?


If it could be demonstrated that any complex organ existed, which could not
possibly have been formed by numerous, successive, slight modifications, my
theory would absolutely break down. But I can find out no such case. No
doubt many organs exist of which we do not know the transitional grades,
more especially if we look to much-isolated species, around which,
according to the theory, there has been much extinction. Or again, if we
take an organ common to all the members of a class, for in this latter case
the organ must have been originally formed at a remote period, since which
all the many members of the class have been developed; and in order to
discover the early transitional grades through which the organ has passed,
we should have to look to very ancient ancestral forms, long since become

We should be extremely cautious in concluding that an organ could not have
been formed by transitional gradations of some kind. Numerous cases could
be given among the lower animals of the same organ performing at the same
time wholly distinct functions; thus in the larva of the dragon-fly and in
the fish Cobites the alimentary canal respires, digests, and excretes. In
the Hydra, the animal may be turned inside out, and the exterior surface
will then digest and the stomach respire. In such cases natural selection
might specialise, if any advantage were thus gained, the whole or part of
an organ, which had previously performed two functions, for one function
alone, and thus by insensible steps greatly change its nature. Many plants
are known which regularly produce at the same time differently constructed
flowers; and if such plants were to produce one kind alone, a great change
would be effected with comparative suddenness in the character of the
species. It is, however, probable that the two sorts of flowers borne by
the same plant were originally differentiated by finely graduated steps,
which may still be followed in some few cases.

Again, two distinct organs, or the same organ under two very different
forms, may simultaneously perform in the same individual the same function,
and this is an extremely important means of transition: to give one
instance--there are fish with gills or branchiae that breathe the air
dissolved in the water, at the same time that they breathe free air in
their swim-bladders, this latter organ being divided by highly vascular
partitions and having a ductus pneumaticus for the supply of air. To give
another instance from the vegetable kingdom: plants climb by three
distinct means, by spirally twining, by clasping a support with their
sensitive tendrils, and by the emission of aerial rootlets; these three
means are usually found in distinct groups, but some few species exhibit
two of the means, or even all three, combined in the same individual. In
all such cases one of the two organs might readily be modified and
perfected so as to perform all the work, being aided during the progress of
modification by the other organ; and then this other organ might be
modified for some other and quite distinct purpose, or be wholly

The illustration of the swim-bladder in fishes is a good one, because it
shows us clearly the highly important fact that an organ originally
constructed for one purpose, namely flotation, may be converted into one
for a widely different purpose, namely respiration. The swim-bladder has,
also, been worked in as an accessory to the auditory organs of certain
fishes. All physiologists admit that the swim-bladder is homologous, or
"ideally similar" in position and structure with the lungs of the higher
vertebrate animals: hence there is no reason to doubt that the swim-
bladder has actually been converted into lungs, or an organ used
exclusively for respiration.

According to this view it may be inferred that all vertebrate animals with
true lungs are descended by ordinary generation from an ancient and unknown
prototype which was furnished with a floating apparatus or swim-bladder.
We can thus, as I infer from Professor Owen's interesting description of
these parts, understand the strange fact that every particle of food and
drink which we swallow has to pass over the orifice of the trachea, with
some risk of falling into the lungs, notwithstanding the beautiful
contrivance by which the glottis is closed. In the higher Vertebrata the
branchiae have wholly disappeared--but in the embryo the slits on the sides
of the neck and the loop-like course of the arteries still mark their
former position. But it is conceivable that the now utterly lost branchiae
might have been gradually worked in by natural selection for some distinct
purpose: for instance, Landois has shown that the wings of insects are
developed from the trachea; it is therefore highly probable that in this
great class organs which once served for respiration have been actually
converted into organs for flight.

In considering transitions of organs, it is so important to bear in mind
the probability of conversion from one function to another, that I will
give another instance. Pedunculated cirripedes have two minute folds of
skin, called by me the ovigerous frena, which serve, through the means of a
sticky secretion, to retain the eggs until they are hatched within the
sack. These cirripedes have no branchiae, the whole surface of the body
and of the sack, together with the small frena, serving for respiration.
The Balanidae or sessile cirripedes, on the other hand, have no ovigerous
frena, the eggs lying loose at the bottom of the sack, within the
well-enclosed shell; but they have, in the same relative position with the
frena, large, much-folded membranes, which freely communicate with the
circulatory lacunae of the sack and body, and which have been considered by
all naturalists to act as branchiae. Now I think no one will dispute that
the ovigerous frena in the one family are strictly homologous with the
branchiae of the other family; indeed, they graduate into each other.
Therefore it need not be doubted that the two little folds of skin, which
originally served as ovigerous frena, but which, likewise, very slightly
aided in the act of respiration, have been gradually converted by natural
selection into branchiae, simply through an increase in their size and the
obliteration of their adhesive glands. If all pedunculated cirripedes had
become extinct, and they have suffered far more extinction than have
sessile cirripedes, who would ever have imagined that the branchiae in this
latter family had originally existed as organs for preventing the ova from
being washed out of the sack?

There is another possible mode of transition, namely, through the
acceleration or retardation of the period of reproduction. This has lately
been insisted on by Professor Cope and others in the United States. It is
now known that some animals are capable of reproduction at a very early
age, before they have acquired their perfect characters; and if this power
became thoroughly well developed in a species, it seems probable that the
adult stage of development would sooner or later be lost; and in this case,
especially if the larva differed much from the mature form, the character
of the species would be greatly changed and degraded. Again, not a few
animals, after arriving at maturity, go on changing in character during
nearly their whole lives. With mammals, for instance, the form of the
skull is often much altered with age, of which Dr. Murie has given some
striking instances with seals. Every one knows how the horns of stags
become more and more branched, and the plumes of some birds become more
finely developed, as they grow older. Professor Cope states that the teeth
of certain lizards change much in shape with advancing years. With
crustaceans not only many trivial, but some important parts assume a new
character, as recorded by Fritz Muller, after maturity. In all such cases-
-and many could be given--if the age for reproduction were retarded, the
character of the species, at least in its adult state, would be modified;
nor is it improbable that the previous and earlier stages of development
would in some cases be hurried through and finally lost. Whether species
have often or ever been modified through this comparatively sudden mode of
transition, I can form no opinion; but if this has occurred, it is probable
that the differences between the young and the mature, and between the
mature and the old, were primordially acquired by graduated steps.


Although we must be extremely cautious in concluding that any organ could
not have been produced by successive, small, transitional gradations, yet
undoubtedly serious cases of difficulty occur.

One of the most serious is that of neuter insects, which are often
differently constructed from either the males or fertile females; but this
case will be treated of in the next chapter. The electric organs of fishes
offer another case of special difficulty; for it is impossible to conceive
by what steps these wondrous organs have been produced. But this is not
surprising, for we do not even know of what use they are. In the gymnotus
and torpedo they no doubt serve as powerful means of defence, and perhaps
for securing prey; yet in the ray, as observed by Matteucci, an analogous
organ in the tail manifests but little electricity, even when the animal is
greatly irritated; so little that it can hardly be of any use for the above
purposes. Moreover, in the ray, besides the organ just referred to, there
is, as Dr. R. McDonnell has shown, another organ near the head, not known
to be electrical, but which appears to be the real homologue of the
electric battery in the torpedo. It is generally admitted that there
exists between these organs and ordinary muscle a close analogy, in
intimate structure, in the distribution of the nerves, and in the manner in
which they are acted on by various reagents. It should, also, be
especially observed that muscular contraction is accompanied by an
electrical discharge; and, as Dr. Radcliffe insists, "in the electrical
apparatus of the torpedo during rest, there would seem to be a charge in
every respect like that which is met with in muscle and nerve during the
rest, and the discharge of the torpedo, instead of being peculiar, may be
only another form of the discharge which attends upon the action of muscle
and motor nerve." Beyond this we cannot at present go in the way of
explanation; but as we know so little about the uses of these organs, and
as we know nothing about the habits and structure of the progenitors of the
existing electric fishes, it would be extremely bold to maintain that no
serviceable transitions are possible by which these organs might have been
gradually developed.

These organs appear at first to offer another and far more serious
difficulty; for they occur in about a dozen kinds of fish, of which several
are widely remote in their affinities. When the same organ is found in
several members of the same class, especially if in members having very
different habits of life, we may generally attribute its presence to
inheritance from a common ancestor; and its absence in some of the members
to loss through disuse or natural selection. So that, if the electric
organs had been inherited from some one ancient progenitor, we might have
expected that all electric fishes would have been specially related to each
other; but this is far from the case. Nor does geology at all lead to the
belief that most fishes formerly possessed electric organs, which their
modified descendants have now lost. But when we look at the subject more
closely, we find in the several fishes provided with electric organs, that
these are situated in different parts of the body, that they differ in
construction, as in the arrangement of the plates, and, according to
Pacini, in the process or means by which the electricity is excited--and
lastly, in being supplied with nerves proceeding from different sources,
and this is perhaps the most important of all the differences. Hence in
the several fishes furnished with electric organs, these cannot be
considered as homologous, but only as analogous in function. Consequently
there is no reason to suppose that they have been inherited from a common
progenitor; for had this been the case they would have closely resembled
each other in all respects. Thus the difficulty of an organ, apparently
the same, arising in several remotely allied species, disappears, leaving
only the lesser yet still great difficulty: namely, by what graduated
steps these organs have been developed in each separate group of fishes.

The luminous organs which occur in a few insects, belonging to widely
different families, and which are situated in different parts of the body,
offer, under our present state of ignorance, a difficulty almost exactly
parallel with that of the electric organs. Other similar cases could be
given; for instance in plants, the very curious contrivance of a mass of
pollen-grains, borne on a foot-stalk with an adhesive gland, is apparently
the same in Orchis and Asclepias, genera almost as remote as is possible
among flowering plants; but here again the parts are not homologous. In
all cases of beings, far removed from each other in the scale of
organisation, which are furnished with similar and peculiar organs, it will
be found that although the general appearance and function of the organs
may be the same, yet fundamental differences between them can always be
detected. For instance, the eyes of Cephalopods or cuttle-fish and of
vertebrate animals appear wonderfully alike; and in such widely sundered
groups no part of this resemblance can be due to inheritance from a common
progenitor. Mr. Mivart has advanced this case as one of special
difficulty, but I am unable to see the force of his argument. An organ for
vision must be formed of transparent tissue, and must include some sort of
lens for throwing an image at the back of a darkened chamber. Beyond this
superficial resemblance, there is hardly any real similarity between the
eyes of cuttle-fish and vertebrates, as may be seen by consulting Hensen's
admirable memoir on these organs in the Cephalopoda. It is impossible for
me here to enter on details, but I may specify a few of the points of
difference. The crystalline lens in the higher cuttle-fish consists of two
parts, placed one behind the other like two lenses, both having a very
different structure and disposition to what occurs in the vertebrata. The
retina is wholly different, with an actual inversion of the elemental
parts, and with a large nervous ganglion included within the membranes of
the eye. The relations of the muscles are as different as it is possible
to conceive, and so in other points. Hence it is not a little difficult to
decide how far even the same terms ought to be employed in describing the
eyes of the Cephalopoda and Vertebrata. It is, of course, open to any one
to deny that the eye in either case could have been developed through the
natural selection of successive slight variations; but if this be admitted
in the one case it is clearly possible in the other; and fundamental
differences of structure in the visual organs of two groups might have been
anticipated, in accordance with this view of their manner of formation. As
two men have sometimes independently hit on the same invention, so in the
several foregoing cases it appears that natural selection, working for the
good of each being, and taking advantage of all favourable variations, has
produced similar organs, as far as function is concerned, in distinct
organic beings, which owe none of their structure in common to inheritance
from a common progenitor.

Fritz Muller, in order to test the conclusions arrived at in this volume,
has followed out with much care a nearly similar line of argument. Several
families of crustaceans include a few species, possessing an air-breathing
apparatus and fitted to live out of the water. In two of these families,
which were more especially examined by Muller, and which are nearly related
to each other, the species agree most closely in all important characters:
namely in their sense organs, circulating systems, in the position of the
tufts of hair within their complex stomachs, and lastly in the whole
structure of the water-breathing branchiae, even to the microscopical hooks
by which they are cleansed. Hence it might have been expected that in the
few species belonging to both families which live on the land, the equally
important air-breathing apparatus would have been the same; for why should
this one apparatus, given for the same purpose, have been made to differ,
while all the other important organs were closely similar, or rather,

Fritz Muller argues that this close similarity in so many points of
structure must, in accordance with the views advanced by me, be accounted
for by inheritance from a common progenitor. But as the vast majority of
the species in the above two families, as well as most other crustaceans,
are aquatic in their habits, it is improbable in the highest degree that
their common progenitor should have been adapted for breathing air. Muller
was thus led carefully to examine the apparatus in the air-breathing
species; and he found it to differ in each in several important points, as
in the position of the orifices, in the manner in which they are opened and
closed, and in some accessory details. Now such differences are
intelligible, and might even have been expected, on the supposition that
species belonging to distinct families had slowly become adapted to live
more and more out of water, and to breathe the air. For these species,
from belonging to distinct families, would have differed to a certain
extent, and in accordance with the principle that the nature of each
variation depends on two factors, viz., the nature of the organism and that
of the surrounding conditions, their variability assuredly would not have
been exactly the same. Consequently natural selection would have had
different materials or variations to work on, in order to arrive at the
same functional result; and the structures thus acquired would almost
necessarily have differed. On the hypothesis of separate acts of creation
the whole case remains unintelligible. This line of argument seems to have
had great weight in leading Fritz Muller to accept the views maintained by
me in this volume.

Another distinguished zoologist, the late Professor Claparede, has argued
in the same manner, and has arrived at the same result. He shows that
there are parasitic mites (Acaridae), belonging to distinct sub-families
and families, which are furnished with hair-claspers. These organs must
have been independently developed, as they could not have been inherited
from a common progenitor; and in the several groups they are formed by the
modification of the fore legs, of the hind legs, of the maxillae or lips,
and of appendages on the under side of the hind part of the body.

In the foregoing cases, we see the same end gained and the same function
performed, in beings not at all or only remotely allied, by organs in
appearance, though not in development, closely similar. On the other hand,
it is a common rule throughout nature that the same end should be gained,
even sometimes in the case of closely related beings, by the most
diversified means. How differently constructed is the feathered wing of a
bird and the membrane-covered wing of a bat; and still more so the four
wings of a butterfly, the two wings of a fly, and the two wings with the
elytra of a beetle. Bivalve shells are made to open and shut, but on what
a number of patterns is the hinge constructed, from the long row of neatly
interlocking teeth in a Nucula to the simple ligament of a Mussel! Seeds
are disseminated by their minuteness, by their capsule being converted into
a light balloon-like envelope, by being embedded in pulp or flesh, formed
of the most diverse parts, and rendered nutritious, as well as
conspicuously coloured, so as to attract and be devoured by birds, by
having hooks and grapnels of many kinds and serrated awns, so as to adhere
to the fur of quadrupeds, and by being furnished with wings and plumes, as
different in shape as they are elegant in structure, so as to be wafted by
every breeze. I will give one other instance: for this subject of the
same end being gained by the most diversified means well deserves
attention. Some authors maintain that organic beings have been formed in
many ways for the sake of mere variety, almost like toys in a shop, but
such a view of nature is incredible. With plants having separated sexes,
and with those in which, though hermaphrodites, the pollen does not
spontaneously fall on the stigma, some aid is necessary for their
fertilisation. With several kinds this is effected by the pollen-grains,
which are light and incoherent, being blown by the wind through mere chance
on to the stigma; and this is the simplest plan which can well be
conceived. An almost equally simple, though very different plan occurs in
many plants in which a symmetrical flower secretes a few drops of nectar,
and is consequently visited by insects; and these carry the pollen from the
anthers to the stigma.

>From this simple stage we may pass through an inexhaustible number of
contrivances, all for the same purpose and effected in essentially the same
manner, but entailing changes in every part of the flower. The nectar may
be stored in variously shaped receptacles, with the stamens and pistils
modified in many ways, sometimes forming trap-like contrivances, and
sometimes capable of neatly adapted movements through irritability or
elasticity. From such structures we may advance till we come to such a
case of extraordinary adaptation as that lately described by Dr. Cruger in
the Coryanthes. This orchid has part of its labellum or lower lip hollowed
out into a great bucket, into which drops of almost pure water continually
fall from two secreting horns which stand above it; and when the bucket is
half-full, the water overflows by a spout on one side. The basal part of
the labellum stands over the bucket, and is itself hollowed out into a sort
of chamber with two lateral entrances; within this chamber there are
curious fleshy ridges. The most ingenious man, if he had not witnessed
what takes place, could never have imagined what purpose all these parts
serve. But Dr. Cruger saw crowds of large humble-bees visiting the
gigantic flowers of this orchid, not in order to suck nectar, but to gnaw
off the ridges within the chamber above the bucket; in doing this they
frequently pushed each other into the bucket, and their wings being thus
wetted they could not fly away, but were compelled to crawl out through the
passage formed by the spout or overflow. Dr. Cruger saw a "continual
procession" of bees thus crawling out of their involuntary bath. The
passage is narrow, and is roofed over by the column, so that a bee, in
forcing its way out, first rubs its back against the viscid stigma and then
against the viscid glands of the pollen-masses. The pollen-masses are thus
glued to the back of the bee which first happens to crawl out through the
passage of a lately expanded flower, and are thus carried away. Dr. Cruger
sent me a flower in spirits of wine, with a bee which he had killed before
it had quite crawled out, with a pollen-mass still fastened to its back.
When the bee, thus provided, flies to another flower, or to the same flower
a second time, and is pushed by its comrades into the bucket and then
crawls out by the passage, the pollen-mass necessarily comes first into
contact with the viscid stigma, and adheres to it, and the flower is
fertilised. Now at last we see the full use of every part of the flower,
of the water-secreting horns of the bucket half-full of water, which
prevents the bees from flying away, and forces them to crawl out through
the spout, and rub against the properly placed viscid pollen-masses and the
viscid stigma.

The construction of the flower in another closely allied orchid, namely,
the Catasetum, is widely different, though serving the same end; and is
equally curious. Bees visit these flowers, like those of the Coryanthes,
in order to gnaw the labellum; in doing this they inevitably touch a long,
tapering, sensitive projection, or, as I have called it, the antenna. This
antenna, when touched, transmits a sensation or vibration to a certain
membrane which is instantly ruptured; this sets free a spring by which the
pollen-mass is shot forth, like an arrow, in the right direction, and
adheres by its viscid extremity to the back of the bee. The pollen-mass of
the male plant (for the sexes are separate in this orchid) is thus carried
to the flower of the female plant, where it is brought into contact with
the stigma, which is viscid enough to break certain elastic threads, and
retain the pollen, thus effecting fertilisation.

How, it may be asked, in the foregoing and in innumerable other instances,
can we understand the graduated scale of complexity and the multifarious
means for gaining the same end. The answer no doubt is, as already
remarked, that when two forms vary, which already differ from each other in
some slight degree, the variability will not be of the same exact nature,
and consequently the results obtained through natural selection for the
same general purpose will not be the same. We should also bear in mind
that every highly developed organism has passed through many changes; and
that each modified structure tends to be inherited, so that each
modification will not readily be quite lost, but may be again and again
further altered. Hence, the structure of each part of each species, for
whatever purpose it may serve, is the sum of many inherited changes,
through which the species has passed during its successive adaptations to
changed habits and conditions of life.

Finally, then, although in many cases it is most difficult even to
conjecture by what transitions organs could have arrived at their present
state; yet, considering how small the proportion of living and known forms
is to the extinct and unknown, I have been astonished how rarely an organ
can be named, towards which no transitional grade is known to lead. It is
certainly true, that new organs appearing as if created for some special
purpose rarely or never appear in any being; as indeed is shown by that
old, but somewhat exaggerated, canon in natural history of "Natura non
facit saltum." We meet with this admission in the writings of almost every
experienced naturalist; or, as Milne Edwards has well expressed it, "Nature
is prodigal in variety, but niggard in innovation." Why, on the theory of
Creation, should there be so much variety and so little real novelty? Why
should all the parts and organs of many independent beings, each supposed
to have been separately created for its own proper place in nature, be so
commonly linked together by graduated steps? Why should not Nature take a
sudden leap from structure to structure? On the theory of natural
selection, we can clearly understand why she should not; for natural
selection acts only by taking advantage of slight successive variations;
she can never take a great and sudden leap, but must advance by the short
and sure, though slow steps.


As natural selection acts by life and death, by the survival of the
fittest, and by the destruction of the less well-fitted individuals, I have
sometimes felt great difficulty in understanding the origin or formation of
parts of little importance; almost as great, though of a very different
kind, as in the case of the most perfect and complex organs.

In the first place, we are much too ignorant in regard to the whole economy
of any one organic being to say what slight modifications would be of
importance or not. In a former chapter I have given instances of very
trifling characters, such as the down on fruit and the colour of its flesh,
the colour of the skin and hair of quadrupeds, which, from being correlated
with constitutional differences, or from determining the attacks of
insects, might assuredly be acted on by natural selection. The tail of the
giraffe looks like an artificially constructed fly-flapper; and it seems at
first incredible that this could have been adapted for its present purpose
by successive slight modifications, each better and better fitted, for so
trifling an object as to drive away flies; yet we should pause before being
too positive even in this case, for we know that the distribution and
existence of cattle and other animals in South America absolutely depend on
their power of resisting the attacks of insects: so that individuals which
could by any means defend themselves from these small enemies, would be
able to range into new pastures and thus gain a great advantage. It is not
that the larger quadrupeds are actually destroyed (except in some rare
cases) by flies, but they are incessantly harassed and their strength
reduced, so that they are more subject to disease, or not so well enabled
in a coming dearth to search for food, or to escape from beasts of prey.

Organs now of trifling importance have probably in some cases been of high
importance to an early progenitor, and, after having been slowly perfected
at a former period, have been transmitted to existing species in nearly the
same state, although now of very slight use; but any actually injurious
deviations in their structure would of course have been checked by natural
selection. Seeing how important an organ of locomotion the tail is in most
aquatic animals, its general presence and use for many purposes in so many
land animals, which in their lungs or modified swim-bladders betray their
aquatic origin, may perhaps be thus accounted for. A well-developed tail
having been formed in an aquatic animal, it might subsequently come to be
worked in for all sorts of purposes, as a fly-flapper, an organ of
prehension, or as an aid in turning, as in the case of the dog, though the
aid in this latter respect must be slight, for the hare, with hardly any
tail, can double still more quickly.

In the second place, we may easily err in attributing importance to
characters, and in believing that they have been developed through natural
selection. We must by no means overlook the effects of the definite action
of changed conditions of life, of so-called spontaneous variations, which
seem to depend in a quite subordinate degree on the nature of the
conditions, of the tendency to reversion to long-lost characters, of the
complex laws of growth, such as of correlation, comprehension, of the
pressure of one part on another, etc., and finally of sexual selection, by
which characters of use to one sex are often gained and then transmitted
more or less perfectly to the other sex, though of no use to the sex. But
structures thus indirectly gained, although at first of no advantage to a
species, may subsequently have been taken advantage of by its modified
descendants, under new conditions of life and newly acquired habits.

If green woodpeckers alone had existed, and we did not know that there were
many black and pied kinds, I dare say that we should have thought that the
green colour was a beautiful adaptation to conceal this tree-frequenting
bird from its enemies; and consequently that it was a character of
importance, and had been acquired through natural selection; as it is, the
colour is probably in chief part due to sexual selection. A trailing palm
in the Malay Archipelago climbs the loftiest trees by the aid of
exquisitely constructed hooks clustered around the ends of the branches,
and this contrivance, no doubt, is of the highest service to the plant; but
as we see nearly similar hooks on many trees which are not climbers, and
which, as there is reason to believe from the distribution of the thorn-
bearing species in Africa and South America, serve as a defence against
browsing quadrupeds, so the spikes on the palm may at first have been
developed for this object, and subsequently have been improved and taken
advantage of by the plant, as it underwent further modification and became
a climber. The naked skin on the head of a vulture is generally considered
as a direct adaptation for wallowing in putridity; and so it may be, or it
may possibly be due to the direct action of putrid matter; but we should be
very cautious in drawing any such inference, when we see that the skin on
the head of the clean-feeding male turkey is likewise naked. The sutures
in the skulls of young mammals have been advanced as a beautiful adaptation
for aiding parturition, and no doubt they facilitate, or may be
indispensable for this act; but as sutures occur in the skulls of young
birds and reptiles, which have only to escape from a broken egg, we may
infer that this structure has arisen from the laws of growth, and has been
taken advantage of in the parturition of the higher animals.

We are profoundly ignorant of the cause of each slight variation or
individual difference; and we are immediately made conscious of this by
reflecting on the differences between the breeds of our domesticated
animals in different countries, more especially in the less civilized
countries, where there has been but little methodical selection. Animals
kept by savages in different countries often have to struggle for their own
subsistence, and are exposed to a certain extent to natural selection, and
individuals with slightly different constitutions would succeed best under
different climates. With cattle susceptibility to the attacks of flies is
correlated with colour, as is the liability to be poisoned by certain
plants; so that even colour would be thus subjected to the action of
natural selection. Some observers are convinced that a damp climate
affects the growth of the hair, and that with the hair the horns are
correlated. Mountain breeds always differ from lowland breeds; and a
mountainous country would probably affect the hind limbs from exercising
them more, and possibly even the form of the pelvis; and then by the law of
homologous variation, the front limbs and the head would probably be
affected. The shape, also, of the pelvis might affect by pressure the
shape of certain parts of the young in the womb. The laborious breathing
necessary in high regions tends, as we have good reason to believe, to
increase the size of the chest; and again correlation would come into play.
The effects of lessened exercise, together with abundant food, on the whole
organisation is probably still more important, and this, as H. von
Nathusius has lately shown in his excellent Treatise, is apparently one
chief cause of the great modification which the breeds of swine have
undergone. But we are far too ignorant to speculate on the relative
importance of the several known and unknown causes of variation; and I have
made these remarks only to show that, if we are unable to account for the
characteristic differences of our several domestic breeds, which
nevertheless are generally admitted to have arisen through ordinary
generation from one or a few parent-stocks, we ought not to lay too much
stress on our ignorance of the precise cause of the slight analogous
differences between true species.


The foregoing remarks lead me to say a few words on the protest lately made
by some naturalists against the utilitarian doctrine that every detail of
structure has been produced for the good of its possessor. They believe
that many structures have been created for the sake of beauty, to delight
man or the Creator (but this latter point is beyond the scope of scientific
discussion), or for the sake of mere variety, a view already discussed.
Such doctrines, if true, would be absolutely fatal to my theory. I fully
admit that many structures are now of no direct use to their possessors,
and may never have been of any use to their progenitors; but this does not
prove that they were formed solely for beauty or variety. No doubt the
definite action of changed conditions, and the various causes of
modifications, lately specified, have all produced an effect, probably a
great effect, independently of any advantage thus gained. But a still more
important consideration is that the chief part of the organisation of every
living creature is due to inheritance; and consequently, though each being
assuredly is well fitted for its place in nature, many structures have now
no very close and direct relation to present habits of life. Thus, we can
hardly believe that the webbed feet of the upland goose, or of the frigate-
bird, are of special use to these birds; we cannot believe that the similar
bones in the arm of the monkey, in the fore leg of the horse, in the wing
of the bat, and in the flipper of the seal, are of special use to these
animals. We may safely attribute these structures to inheritance. But
webbed feet no doubt were as useful to the progenitor of the upland goose
and of the frigate-bird, as they now are to the most aquatic of living
birds. So we may believe that the progenitor of the seal did not possess a
flipper, but a foot with five toes fitted for walking or grasping; and we
may further venture to believe that the several bones in the limbs of the
monkey, horse and bat, were originally developed, on the principle of
utility, probably through the reduction of more numerous bones in the fin
of some ancient fish-like progenitor of the whole class. It is scarcely
possible to decide how much allowance ought to be made for such causes of
change, as the definite action of external conditions, so-called
spontaneous variations, and the complex laws of growth; but with these
important exceptions, we may conclude that the structure of every living
creature either now is, or was formerly, of some direct or indirect use to
its possessor.

With respect to the belief that organic beings have been created beautiful
for the delight of man--a belief which it has been pronounced is subversive
of my whole theory--I may first remark that the sense of beauty obviously
depends on the nature of the mind, irrespective of any real quality in the
admired object; and that the idea of what is beautiful, is not innate or
unalterable. We see this, for instance, in the men of different races
admiring an entirely different standard of beauty in their women. If
beautiful objects had been created solely for man's gratification, it ought
to be shown that before man appeared there was less beauty on the face of
the earth than since he came on the stage. Were the beautiful volute and
cone shells of the Eocene epoch, and the gracefully sculptured ammonites of
the Secondary period, created that man might ages afterwards admire them in
his cabinet? Few objects are more beautiful than the minute siliceous
cases of the diatomaceae: were these created that they might be examined
and admired under the higher powers of the microscope? The beauty in this
latter case, and in many others, is apparently wholly due to symmetry of
growth. Flowers rank among the most beautiful productions of nature; but
they have been rendered conspicuous in contrast with the green leaves, and
in consequence at the same time beautiful, so that they may be easily
observed by insects. I have come to this conclusion from finding it an
invariable rule that when a flower is fertilised by the wind it never has a
gaily-coloured corolla. Several plants habitually produce two kinds of
flowers; one kind open and coloured so as to attract insects; the other
closed, not coloured, destitute of nectar, and never visited by insects.
Hence, we may conclude that, if insects had not been developed on the face
of the earth, our plants would not have been decked with beautiful flowers,
but would have produced only such poor flowers as we see on our fir, oak,
nut and ash trees, on grasses, spinach, docks and nettles, which are all
fertilised through the agency of the wind. A similar line of argument
holds good with fruits; that a ripe strawberry or cherry is as pleasing to
the eye as to the palate--that the gaily-coloured fruit of the spindle-wood
tree and the scarlet berries of the holly are beautiful objects--will be
admitted by everyone. But this beauty serves merely as a guide to birds
and beasts, in order that the fruit may be devoured and the matured seeds
disseminated. I infer that this is the case from having as yet found no
exception to the rule that seeds are always thus disseminated when embedded
within a fruit of any kind (that is within a fleshy or pulpy envelope), if
it be coloured of any brilliant tint, or rendered conspicuous by being
white or black.

On the other hand, I willingly admit that a great number of male animals,
as all our most gorgeous birds, some fishes, reptiles, and mammals, and a
host of magnificently coloured butterflies, have been rendered beautiful
for beauty's sake. But this has been effected through sexual selection,
that is, by the more beautiful males having been continually preferred by
the females, and not for the delight of man. So it is with the music of
birds. We may infer from all this that a nearly similar taste for
beautiful colours and for musical sounds runs through a large part of the
animal kingdom. When the female is as beautifully coloured as the male,
which is not rarely the case with birds and butterflies, the cause
apparently lies in the colours acquired through sexual selection having
been transmitted to both sexes, instead of to the males alone. How the
sense of beauty in its simplest form--that is, the reception of a peculiar
kind of pleasure from certain colours, forms and sounds--was first
developed in the mind of man and of the lower animals, is a very obscure
subject. The same sort of difficulty is presented if we enquire how it is
that certain flavours and odours give pleasure, and others displeasure.
Habit in all these cases appears to have come to a certain extent into
play; but there must be some fundamental cause in the constitution of the
nervous system in each species.

Natural selection cannot possibly produce any modification in a species
exclusively for the good of another species; though throughout nature one
species incessantly takes advantage of, and profits by the structures of
others. But natural selection can and does often produce structures for
the direct injury of other animals, as we see in the fang of the adder, and
in the ovipositor of the ichneumon, by which its eggs are deposited in the
living bodies of other insects. If it could be proved that any part of the
structure of any one species had been formed for the exclusive good of
another species, it would annihilate my theory, for such could not have
been produced through natural selection. Although many statements may be
found in works on natural history to this effect, I cannot find even one
which seems to me of any weight. It is admitted that the rattlesnake has a
poison-fang for its own defence and for the destruction of its prey; but
some authors suppose that at the same time it is furnished with a rattle
for its own injury, namely, to warn its prey. I would almost as soon
believe that the cat curls the end of its tail when preparing to spring, in
order to warn the doomed mouse. It is a much more probable view that the
rattlesnake uses its rattle, the cobra expands its frill and the puff-adder
swells while hissing so loudly and harshly, in order to alarm the many
birds and beasts which are known to attack even the most venomous species.
Snakes act on the same principle which makes the hen ruffle her feathers
and expand her wings when a dog approaches her chickens. But I have not
space here to enlarge on the many ways by which animals endeavour to
frighten away their enemies.

Natural selection will never produce in a being any structure more
injurious than beneficial to that being, for natural selection acts solely
by and for the good of each. No organ will be formed, as Paley has
remarked, for the purpose of causing pain or for doing an injury to its
possessor. If a fair balance be struck between the good and evil caused by
each part, each will be found on the whole advantageous. After the lapse
of time, under changing conditions of life, if any part comes to be
injurious, it will be modified; or if it be not so, the being will become
extinct, as myriads have become extinct.

Natural selection tends only to make each organic being as perfect as, or
slightly more perfect than the other inhabitants of the same country with
which it comes into competition. And we see that this is the standard of
perfection attained under nature. The endemic productions of New Zealand,
for instance, are perfect, one compared with another; but they are now
rapidly yielding before the advancing legions of plants and animals
introduced from Europe. Natural selection will not produce absolute
perfection, nor do we always meet, as far as we can judge, with this high
standard under nature. The correction for the aberration of light is said
by Muller not to be perfect even in that most perfect organ, the human eye.
Helmholtz, whose judgment no one will dispute, after describing in the
strongest terms the wonderful powers of the human eye, adds these
remarkable words: "That which we have discovered in the way of inexactness
and imperfection in the optical machine and in the image on the retina, is
as nothing in comparison with the incongruities which we have just come
across in the domain of the sensations. One might say that nature has
taken delight in accumulating contradictions in order to remove all
foundation from the theory of a pre-existing harmony between the external
and internal worlds." If our reason leads us to admire with enthusiasm a
multitude of inimitable contrivances in nature, this same reason tells us,
though we may easily err on both sides, that some other contrivances are
less perfect. Can we consider the sting of the bee as perfect, which, when
used against many kinds of enemies, cannot be withdrawn, owing to the
backward serratures, and thus inevitably causes the death of the insect by
tearing out its viscera?

If we look at the sting of the bee, as having existed in a remote
progenitor, as a boring and serrated instrument, like that in so many
members of the same great order, and that it has since been modified but
not perfected for its present purpose, with the poison originally adapted
for some other object, such as to produce galls, since intensified, we can
perhaps understand how it is that the use of the sting should so often
cause the insect's own death: for if on the whole the power of stinging be
useful to the social community, it will fulfil all the requirements of
natural selection, though it may cause the death of some few members. If
we admire the truly wonderful power of scent by which the males of many
insects find their females, can we admire the production for this single
purpose of thousands of drones, which are utterly useless to the community
for any other purpose, and which are ultimately slaughtered by their
industrious and sterile sisters? It may be difficult, but we ought to
admire the savage instinctive hatred of the queen-bee, which urges her to
destroy the young queens, her daughters, as soon as they are born, or to
perish herself in the combat; for undoubtedly this is for the good of the
community; and maternal love or maternal hatred, though the latter
fortunately is most rare, is all the same to the inexorable principles of
natural selection. If we admire the several ingenious contrivances by
which orchids and many other plants are fertilised through insect agency,
can we consider as equally perfect the elaboration of dense clouds of
pollen by our fir-trees, so that a few granules may be wafted by chance on
to the ovules?


We have in this chapter discussed some of the difficulties and objections
which may be urged against the theory. Many of them are serious; but I
think that in the discussion light has been thrown on several facts, which
on the belief of independent acts of creation are utterly obscure. We have
seen that species at any one period are not indefinitely variable, and are
not linked together by a multitude of intermediate gradations, partly
because the process of natural selection is always very slow, and at any
one time acts only on a few forms; and partly because the very process of
natural selection implies the continual supplanting and extinction of
preceding and intermediate gradations. Closely allied species, now living
on a continuous area, must often have been formed when the area was not
continuous, and when the conditions of life did not insensibly graduate
away from one part to another. When two varieties are formed in two
districts of a continuous area, an intermediate variety will often be
formed, fitted for an intermediate zone; but from reasons assigned, the
intermediate variety will usually exist in lesser numbers than the two
forms which it connects; consequently the two latter, during the course of
further modification, from existing in greater numbers, will have a great
advantage over the less numerous intermediate variety, and will thus
generally succeed in supplanting and exterminating it.

We have seen in this chapter how cautious we should be in concluding that
the most different habits of life could not graduate into each other; that
a bat, for instance, could not have been formed by natural selection from
an animal which at first only glided through the air.

We have seen that a species under new conditions of life may change its
habits, or it may have diversified habits, with some very unlike those of
its nearest congeners. Hence we can understand, bearing in mind that each
organic being is trying to live wherever it can live, how it has arisen
that there are upland geese with webbed feet, ground woodpeckers, diving
thrushes, and petrels with the habits of auks.

Although the belief that an organ so perfect as the eye could have been
formed by natural selection, is enough to stagger any one; yet in the case
of any organ, if we know of a long series of gradations in complexity, each
good for its possessor, then under changing conditions of life, there is no
logical impossibility in the acquirement of any conceivable degree of
perfection through natural selection. In the cases in which we know of no
intermediate or transitional states, we should be extremely cautious in
concluding that none can have existed, for the metamorphoses of many organs
show what wonderful changes in function are at least possible. For
instance, a swim-bladder has apparently been converted into an
air-breathing lung. The same organ having performed simultaneously very
different functions, and then having been in part or in whole specialised
for one function; and two distinct organs having performed at the same time
the same function, the one having been perfected whilst aided by the other,
must often have largely facilitated transitions.

We have seen that in two beings widely remote from each other in the
natural scale, organs serving for the same purpose and in external
appearance closely similar may have been separately and independently
formed; but when such organs are closely examined, essential differences in
their structure can almost always be detected; and this naturally follows
from the principle of natural selection. On the other hand, the common
rule throughout nature is infinite diversity of structure for gaining the
same end; and this again naturally follows from the same great principle.

In many cases we are far too ignorant to be enabled to assert that a part
or organ is so unimportant for the welfare of a species, that modifications
in its structure could not have been slowly accumulated by means of natural
selection. In many other cases, modifications are probably the direct
result of the laws of variation or of growth, independently of any good
having been thus gained. But even such structures have often, as we may
feel assured, been subsequently taken advantage of, and still further
modified, for the good of species under new conditions of life. We may,
also, believe that a part formerly of high importance has frequently been
retained (as the tail of an aquatic animal by its terrestrial descendants),
though it has become of such small importance that it could not, in its
present state, have been acquired by means of natural selection.

Natural selection can produce nothing in one species for the exclusive good
or injury of another; though it may well produce parts, organs, and
excretions highly useful or even indispensable, or highly injurious to
another species, but in all cases at the same time useful to the possessor.
In each well-stocked country natural selection acts through the competition
of the inhabitants and consequently leads to success in the battle for
life, only in accordance with the standard of that particular country.
Hence the inhabitants of one country, generally the smaller one, often
yield to the inhabitants of another and generally the larger country. For
in the larger country there will have existed more individuals, and more
diversified forms, and the competition will have been severer, and thus the
standard of perfection will have been rendered higher. Natural selection
will not necessarily lead to absolute perfection; nor, as far as we can
judge by our limited faculties, can absolute perfection be everywhere

On the theory of natural selection we can clearly understand the full
meaning of that old canon in natural history, "Natura non facit saltum."
This canon, if we look to the present inhabitants alone of the world, is
not strictly correct; but if we include all those of past times, whether
known or unknown, it must on this theory be strictly true.

It is generally acknowledged that all organic beings have been formed on
two great laws--Unity of Type, and the Conditions of Existence. By unity
of type is meant that fundamental agreement in structure which we see in
organic beings of the same class, and which is quite independent of their
habits of life. On my theory, unity of type is explained by unity of
descent. The expression of conditions of existence, so often insisted on
by the illustrious Cuvier, is fully embraced by the principle of natural
selection. For natural selection acts by either now adapting the varying
parts of each being to its organic and inorganic conditions of life; or by
having adapted them during past periods of time: the adaptations being
aided in many cases by the increased use or disuse of parts, being affected
by the direct action of external conditions of life, and subjected in all
cases to the several laws of growth and variation. Hence, in fact, the law
of the Conditions of Existence is the higher law; as it includes, through
the inheritance of former variations and adaptations, that of Unity of



Longevity -- Modifications not necessarily simultaneous -- Modifications
apparently of no direct service -- Progressive development -- Characters of
small functional importance, the most constant -- Supposed incompetence of
natural selection to account for the incipient stages of useful structures
-- Causes which interfere with the acquisition through natural selection of
useful structures -- Gradations of structure with changed functions --
Widely different organs in members of the same class, developed from one
and the same source -- Reasons for disbelieving in great and abrupt

I will devote this chapter to the consideration of various miscellaneous
objections which have been advanced against my views, as some of the
previous discussions may thus be made clearer; but it would be useless to
discuss all of them, as many have been made by writers who have not taken
the trouble to understand the subject. Thus a distinguished German
naturalist has asserted that the weakest part of my theory is, that I
consider all organic beings as imperfect: what I have really said is, that
all are not as perfect as they might have been in relation to their
conditions; and this is shown to be the case by so many native forms in
many quarters of the world having yielded their places to intruding
foreigners. Nor can organic beings, even if they were at any one time
perfectly adapted to their conditions of life, have remained so, when their
conditions changed, unless they themselves likewise changed; and no one
will dispute that the physical conditions of each country, as well as the
number and kinds of its inhabitants, have undergone many mutations.

A critic has lately insisted, with some parade of mathematical accuracy,
that longevity is a great advantage to all species, so that he who believes
in natural selection "must arrange his genealogical tree" in such a manner
that all the descendants have longer lives than their progenitors! Cannot
our critics conceive that a biennial plant or one of the lower animals
might range into a cold climate and perish there every winter; and yet,
owing to advantages gained through natural selection, survive from year to
year by means of its seeds or ova? Mr. E. Ray Lankester has recently
discussed this subject, and he concludes, as far as its extreme complexity
allows him to form a judgment, that longevity is generally related to the
standard of each species in the scale of organisation, as well as to the
amount of expenditure in reproduction and in general activity. And these
conditions have, it is probable, been largely determined through natural

It has been argued that, as none of the animals and plants of Egypt, of
which we know anything, have changed during the last three or four thousand
years, so probably have none in any part of the world. But, as Mr. G.H.
Lewes has remarked, this line of argument proves too much, for the ancient
domestic races figured on the Egyptian monuments, or embalmed, are closely
similar or even identical with those now living; yet all naturalists admit
that such races have been produced through the modification of their
original types. The many animals which have remained unchanged since the
commencement of the glacial period, would have been an incomparably
stronger case, for these have been exposed to great changes of climate and
have migrated over great distances; whereas, in Egypt, during the last
several thousand years, the conditions of life, as far as we know, have
remained absolutely uniform. The fact of little or no modification having
been effected since the glacial period, would have been of some avail
against those who believe in an innate and necessary law of development,
but is powerless against the doctrine of natural selection or the survival
of the fittest, which implies that when variations or individual
differences of a beneficial nature happen to arise, these will be
preserved; but this will be effected only under certain favourable

The celebrated palaeontologist, Bronn, at the close of his German
translation of this work, asks how, on the principle of natural selection,
can a variety live side by side with the parent species? If both have
become fitted for slightly different habits of life or conditions, they
might live together; and if we lay on one side polymorphic species, in
which the variability seems to be of a peculiar nature, and all mere
temporary variations, such as size, albinism, etc., the more permanent
varieties are generally found, as far as I can discover, inhabiting
distinct stations, such as high land or low land, dry or moist districts.
Moreover, in the case of animals which wander much about and cross freely,
their varieties seem to be generally confined to distinct regions.

Bronn also insists that distinct species never differ from each other in
single characters, but in many parts; and he asks, how it always comes that
many parts of the organisation should have been modified at the same time
through variation and natural selection? But there is no necessity for
supposing that all the parts of any being have been simultaneously
modified. The most striking modifications, excellently adapted for some
purpose, might, as was formerly remarked, be acquired by successive
variations, if slight, first in one part and then in another; and as they
would be transmitted all together, they would appear to us as if they had
been simultaneously developed. The best answer, however, to the above
objection is afforded by those domestic races which have been modified,
chiefly through man's power of selection, for some special purpose. Look
at the race and dray-horse, or at the greyhound and mastiff. Their whole
frames, and even their mental characteristics, have been modified; but if
we could trace each step in the history of their transformation--and the
latter steps can be traced--we should not see great and simultaneous
changes, but first one part and then another slightly modified and
improved. Even when selection has been applied by man to some one
character alone--of which our cultivated plants offer the best instances--
it will invariably be found that although this one part, whether it be the
flower, fruit, or leaves, has been greatly changed, almost all the other
parts have been slightly modified. This may be attributed partly to the
principle of correlated growth, and partly to so-called spontaneous

A much more serious objection has been urged by Bronn, and recently by
Broca, namely, that many characters appear to be of no service whatever to
their possessors, and therefore cannot have been influenced through natural
selection. Bronn adduces the length of the ears and tails in the different
species of hares and mice--the complex folds of enamel in the teeth of many
animals, and a multitude of analogous cases. With respect to plants, this
subject has been discussed by Nageli in an admirable essay. He admits that
natural selection has effected much, but he insists that the families of
plants differ chiefly from each other in morphological characters, which
appear to be quite unimportant for the welfare of the species. He
consequently believes in an innate tendency towards progressive and more
perfect development. He specifies the arrangement of the cells in the
tissues, and of the leaves on the axis, as cases in which natural selection
could not have acted. To these may be added the numerical divisions in the
parts of the flower, the position of the ovules, the shape of the seed,
when not of any use for dissemination, etc.

There is much force in the above objection. Nevertheless, we ought, in the
first place, to be extremely cautious in pretending to decide what
structures now are, or have formerly been, of use to each species. In the
second place, it should always be borne in mind that when one part is
modified, so will be other parts, through certain dimly seen causes, such
as an increased or diminished flow of nutriment to a part, mutual pressure,
an early developed part affecting one subsequently developed, and so forth
--as well as through other causes which lead to the many mysterious cases
of correlation, which we do not in the least understand. These agencies
may be all grouped together, for the sake of brevity, under the expression
of the laws of growth. In the third place, we have to allow for the direct
and definite action of changed conditions of life, and for so-called
spontaneous variations, in which the nature of the conditions apparently
plays a quite subordinate part. Bud-variations, such as the appearance of
a moss-rose on a common rose, or of a nectarine on a peach-tree, offer good
instances of spontaneous variations; but even in these cases, if we bear in
mind the power of a minute drop of poison in producing complex galls, we
ought not to feel too sure that the above variations are not the effect of
some local change in the nature of the sap, due to some change in the
conditions. There must be some efficient cause for each slight individual
difference, as well as for more strongly marked variations which
occasionally arise; and if the unknown cause were to act persistently, it
is almost certain that all the individuals of the species would be
similarly modified.

In the earlier editions of this work I underrated, as it now seems
probable, the frequency and importance of modifications due to spontaneous
variability. But it is impossible to attribute to this cause the
innumerable structures which are so well adapted to the habits of life of
each species. I can no more believe in this than that the well-adapted
form of a race-horse or greyhound, which before the principle of selection
by man was well understood, excited so much surprise in the minds of the
older naturalists, can thus be explained.

It may be worth while to illustrate some of the foregoing remarks. With
respect to the assumed inutility of various parts and organs, it is hardly
necessary to observe that even in the higher and best-known animals many
structures exist, which are so highly developed that no one doubts that
they are of importance, yet their use has not been, or has only recently
been, ascertained. As Bronn gives the length of the ears and tail in the
several species of mice as instances, though trifling ones, of differences
in structure which can be of no special use, I may mention that, according
to Dr. Schobl, the external ears of the common mouse are supplied in an
extraordinary manner with nerves, so that they no doubt serve as tactile
organs; hence the length of the ears can hardly be quite unimportant. We
shall, also, presently see that the tail is a highly useful prehensile
organ to some of the species; and its use would be much influence by its

With respect to plants, to which on account of Nageli's essay I shall
confine myself in the following remarks, it will be admitted that the
flowers of the orchids present a multitude of curious structures, which a
few years ago would have been considered as mere morphological differences
without any special function; but they are now known to be of the highest
importance for the fertilisation of the species through the aid of insects,
and have probably been gained through natural selection. No one until
lately would have imagined that in dimorphic and trimorphic plants the
different lengths of the stamens and pistils, and their arrangement, could
have been of any service, but now we know this to be the case.

In certain whole groups of plants the ovules stand erect, and in others
they are suspended; and within the same ovarium of some few plants, one
ovule holds the former and a second ovule the latter position. These
positions seem at first purely morphological, or of no physiological
signification; but Dr. Hooker informs me that within the same ovarium the
upper ovules alone in some cases, and in others the lower ones alone are
fertilised; and he suggests that this probably depends on the direction in
which the pollen-tubes enter the ovarium. If so, the position of the
ovules, even when one is erect and the other suspended within the same
ovarium, would follow the selection of any slight deviations in position
which favoured their fertilisation, and the production of seed.

Several plants belonging to distinct orders habitually produce flowers of
two kinds--the one open, of the ordinary structure, the other closed and
imperfect. These two kinds of flowers sometimes differ wonderfully in
structure, yet may be seen to graduate into each other on the same plant.
The ordinary and open flowers can be intercrossed; and the benefits which
certainly are derived from this process are thus secured. The closed and
imperfect flowers are, however, manifestly of high importance, as they
yield with the utmost safety a large stock of seed, with the expenditure of
wonderfully little pollen. The two kinds of flowers often differ much, as
just stated, in structure. The petals in the imperfect flowers almost
always consist of mere rudiments, and the pollen-grains are reduced in
diameter. In Ononis columnae five of the alternate stamens are
rudimentary; and in some species of Viola three stamens are in this state,
two retaining their proper function, but being of very small size. In six
out of thirty of the closed flowers in an Indian violet (name unknown, for
the plants have never produced with me perfect flowers), the sepals are
reduced from the normal number of five to three. In one section of the
Malpighiaceae the closed flowers, according to A. de Jussieu, are still
further modified, for the five stamens which stand opposite to the sepals
are all aborted, a sixth stamen standing opposite to a petal being alone
developed; and this stamen is not present in the ordinary flowers of this
species; the style is aborted; and the ovaria are reduced from three to
two. Now although natural selection may well have had the power to prevent
some of the flowers from expanding, and to reduce the amount of pollen,
when rendered by the closure of the flowers superfluous, yet hardly any of
the above special modifications can have been thus determined, but must
have followed from the laws of growth, including the functional inactivity
of parts, during the progress of the reduction of the pollen and the
closure of the flowers.

It is so necessary to appreciate the important effects of the laws of
growth, that I will give some additional cases of another kind, namely of
differences in the same part or organ, due to differences in relative
position on the same plant. In the Spanish chestnut, and in certain fir-
trees, the angles of divergence of the leaves differ, according to Schacht,
in the nearly horizontal and in the upright branches. In the common rue
and some other plants, one flower, usually the central or terminal one,
opens first, and has five sepals and petals, and five divisions to the
ovarium; while all the other flowers on the plant are tetramerous. In the
British Adoxa the uppermost flower generally has two calyx-lobes with the
other organs tetramerous, while the surrounding flowers generally have
three calyx-lobes with the other organs pentamerous. In many Compositae
and Umbelliferae (and in some other plants) the circumferential flowers
have their corollas much more developed than those of the centre; and this
seems often connected with the abortion of the reproductive organs. It is
a more curious fact, previously referred to, that the achenes or seeds of
the circumference and centre sometimes differ greatly in form, colour and
other characters. In Carthamus and some other Compositae the central
achenes alone are furnished with a pappus; and in Hyoseris the same head
yields achenes of three different forms. In certain Umbelliferae the
exterior seeds, according to Tausch, are orthospermous, and the central one
coelospermous, and this is a character which was considered by De Candolle
to be in other species of the highest systematic importance. Professor
Braun mentions a Fumariaceous genus, in which the flowers in the lower part
of the spike bear oval, ribbed, one-seeded nutlets; and in the upper part
of the spike, lanceolate, two-valved and two-seeded siliques. In these
several cases, with the exception of that of the well-developed ray-
florets, which are of service in making the flowers conspicuous to insects,
natural selection cannot, as far as we can judge, have come into play, or
only in a quite subordinate manner. All these modifications follow from
the relative position and inter-action of the parts; and it can hardly be
doubted that if all the flowers and leaves on the same plant had been
subjected to the same external and internal condition, as are the flowers
and leaves in certain positions, all would have been modified in the same

In numerous other cases we find modifications of structure, which are
considered by botanists to be generally of a highly important nature,
affecting only some of the flowers on the same plant, or occurring on
distinct plants, which grow close together under the same conditions. As
these variations seem of no special use to the plants, they cannot have
been influenced by natural selection. Of their cause we are quite
ignorant; we cannot even attribute them, as in the last class of cases, to
any proximate agency, such as relative position. I will give only a few
instances. It is so common to observe on the same plant, flowers
indifferently tetramerous, pentamerous, etc., that I need not give
examples; but as numerical variations are comparatively rare when the parts
are few, I may mention that, according to De Candolle, the flowers of
Papaver bracteatum offer either two sepals with four petals (which is the
common type with poppies), or three sepals with six petals. The manner in
which the petals are folded in the bud is in most groups a very constant
morphological character; but Professor Asa Gray states that with some
species of Mimulus, the aestivation is almost as frequently that of the
Rhinanthideae as of the Antirrhinideae, to which latter tribe the genus
belongs. Aug. St. Hilaire gives the following cases: the genus
Zanthoxylon belongs to a division of the Rutaceae with a single ovary, but
in some species flowers may be found on the same plant, and even in the
same panicle, with either one or two ovaries. In Helianthemum the capsule
has been described as unilocular or tri-locular; and in H. mutabile, "Une
lame PLUS OU MOINS LARGE, s'etend entre le pericarpe et le placenta." In
the flowers of Saponaria officinalis Dr. Masters has observed instances of
both marginal and free central placentation. Lastly, St. Hilaire found
towards the southern extreme of the range of Gomphia oleaeformis two forms
which he did not at first doubt were distinct species, but he subsequently
saw them growing on the same bush; and he then adds, "Voila donc dans un
meme individu des loges et un style qui se rattachent tantot a un axe
verticale et tantot a un gynobase."

We thus see that with plants many morphological changes may be attributed
to the laws of growth and the inter-action of parts, independently of
natural selection. But with respect to Nageli's doctrine of an innate
tendency towards perfection or progressive development, can it be said in
the case of these strongly pronounced variations, that the plants have been
caught in the act of progressing towards a higher state of development? On
the contrary, I should infer from the mere fact of the parts in question
differing or varying greatly on the same plant, that such modifications
were of extremely small importance to the plants themselves, of whatever
importance they may generally be to us for our classifications. The
acquisition of a useless part can hardly be said to raise an organism in
the natural scale; and in the case of the imperfect, closed flowers, above
described, if any new principle has to be invoked, it must be one of
retrogression rather than of progression; and so it must be with many
parasitic and degraded animals. We are ignorant of the exciting cause of
the above specified modifications; but if the unknown cause were to act
almost uniformly for a length of time, we may infer that the result would
be almost uniform; and in this case all the individuals of the species
would be modified in the same manner.

>From the fact of the above characters being unimportant for the welfare of
the species, any slight variations which occurred in them would not have
been accumulated and augmented through natural selection. A structure
which has been developed through long-continued selection, when it ceases
to be of service to a species, generally becomes variable, as we see with
rudimentary organs; for it will no longer be regulated by this same power
of selection. But when, from the nature of the organism and of the
conditions, modifications have been induced which are unimportant for the
welfare of the species, they may be, and apparently often have been,
transmitted in nearly the same state to numerous, otherwise modified,
descendants. It cannot have been of much importance to the greater number
of mammals, birds, or reptiles, whether they were clothed with hair,
feathers or scales; yet hair has been transmitted to almost all mammals,
feathers to all birds, and scales to all true reptiles. A structure,
whatever it may be, which is common to many allied forms, is ranked by us
as of high systematic importance, and consequently is often assumed to be
of high vital importance to the species. Thus, as I am inclined to
believe, morphological differences, which we consider as important--such as
the arrangement of the leaves, the divisions of the flower or of the
ovarium, the position of the ovules, etc., first appeared in many cases as
fluctuating variations, which sooner or later became constant through the
nature of the organism and of the surrounding conditions, as well as
through the intercrossing of distinct individuals, but not through natural
selection; for as these morphological characters do not affect the welfare
of the species, any slight deviations in them could not have been governed
or accumulated through this latter agency. It is a strange result which we
thus arrive at, namely, that characters of slight vital importance to the
species, are the most important to the systematist; but, as we shall
hereafter see when we treat of the genetic principle of classification,
this is by no means so paradoxical as it may at first appear.

Although we have no good evidence of the existence in organic beings of an
innate tendency towards progressive development, yet this necessarily
follows, as I have attempted to show in the fourth chapter, through the
continued action of natural selection. For the best definition which has
ever been given of a high standard of organisation, is the degree to which
the parts have been specialised or differentiated; and natural selection
tends towards this end, inasmuch as the parts are thus enabled to perform
their functions more efficiently.

A distinguished zoologist, Mr. St. George Mivart, has recently collected
all the objections which have ever been advanced by myself and others
against the theory of natural selection, as propounded by Mr. Wallace and
myself, and has illustrated them with admirable art and force. When thus
marshalled, they make a formidable array; and as it forms no part of Mr.
Mivart's plan to give the various facts and considerations opposed to his
conclusions, no slight effort of reason and memory is left to the reader,
who may wish to weigh the evidence on both sides. When discussing special
cases, Mr. Mivart passes over the effects of the increased use and disuse
of parts, which I have always maintained to be highly important, and have
treated in my "Variation under Domestication" at greater length than, as I
believe, any other writer. He likewise often assumes that I attribute
nothing to variation, independently of natural selection, whereas in the
work just referred to I have collected a greater number of well-established
cases than can be found in any other work known to me. My judgment may not
be trustworthy, but after reading with care Mr. Mivart's book, and
comparing each section with what I have said on the same head, I never
before felt so strongly convinced of the general truth of the conclusions
here arrived at, subject, of course, in so intricate a subject, to much
partial error.

All Mr. Mivart's objections will be, or have been, considered in the
present volume. The one new point which appears to have struck many
readers is, "That natural selection is incompetent to account for the
incipient stages of useful structures." This subject is intimately
connected with that of the gradation of the characters, often accompanied
by a change of function, for instance, the conversion of a swim-bladder
into lungs, points which were discussed in the last chapter under two
headings. Nevertheless, I will here consider in some detail several of the
cases advanced by Mr. Mivart, selecting those which are the most
illustrative, as want of space prevents me from considering all.

The giraffe, by its lofty stature, much elongated neck, fore legs, head and
tongue, has its whole frame beautifully adapted for browsing on the higher
branches of trees. It can thus obtain food beyond the reach of the other
Ungulata or hoofed animals inhabiting the same country; and this must be a
great advantage to it during dearths. The Niata cattle in South America
show us how small a difference in structure may make, during such periods,
a great difference in preserving an animal's life. These cattle can browse
as well as others on grass, but from the projection of the lower jaw they
cannot, during the often recurrent droughts, browse on the twigs of trees,
reeds, etc., to which food the common cattle and horses are then driven; so
that at these times the Niatas perish, if not fed by their owners. Before
coming to Mr. Mivart's objections, it may be well to explain once again how
natural selection will act in all ordinary cases. Man has modified some of
his animals, without necessarily having attended to special points of
structure, by simply preserving and breeding from the fleetest individuals,
as with the race-horse and greyhound, or as with the game-cock, by breeding
from the victorious birds. So under nature with the nascent giraffe, the
individuals which were the highest browsers and were able during dearths to
reach even an inch or two above the others, will often have been preserved;
for they will have roamed over the whole country in search of food. That
the individuals of the same species often differ slightly in the relative
lengths of all their parts may be seen in many works of natural history, in
which careful measurements are given. These slight proportional
differences, due to the laws of growth and variation, are not of the
slightest use or importance to most species. But it will have been
otherwise with the nascent giraffe, considering its probable habits of
life; for those individuals which had some one part or several parts of
their bodies rather more elongated than usual, would generally have
survived. These will have intercrossed and left offspring, either
inheriting the same bodily peculiarities, or with a tendency to vary again
in the same manner; while the individuals less favoured in the same
respects will have been the most liable to perish.

We here see that there is no need to separate single pairs, as man does,
when he methodically improves a breed: natural selection will preserve and
thus separate all the superior individuals, allowing them freely to
intercross, and will destroy all the inferior individuals. By this process
long-continued, which exactly corresponds with what I have called
unconscious selection by man, combined, no doubt, in a most important
manner with the inherited effects of the increased use of parts, it seems
to me almost certain that an ordinary hoofed quadruped might be converted
into a giraffe.

To this conclusion Mr. Mivart brings forward two objections. One is that
the increased size of the body would obviously require an increased supply
of food, and he considers it as "very problematical whether the
disadvantages thence arising would not, in times of scarcity, more than
counterbalance the advantages." But as the giraffe does actually exist in
large numbers in Africa, and as some of the largest antelopes in the world,
taller than an ox, abound there, why should we doubt that, as far as size
is concerned, intermediate gradations could formerly have existed there,
subjected as now to severe dearths. Assuredly the being able to reach, at
each stage of increased size, to a supply of food, left untouched by the
other hoofed quadrupeds of the country, would have been of some advantage
to the nascent giraffe. Nor must we overlook the fact, that increased bulk
would act as a protection against almost all beasts of prey excepting the
lion; and against this animal, its tall neck--and the taller the better--
would, as Mr. Chauncey Wright has remarked, serve as a watch-tower. It is
from this cause, as Sir S. Baker remarks, that no animal is more difficult
to stalk than the giraffe. This animal also uses its long neck as a means
of offence or defence, by violently swinging its head armed with stump-like
horns. The preservation of each species can rarely be determined by any
one advantage, but by the union of all, great and small.

Mr. Mivart then asks (and this is his second objection), if natural
selection be so potent, and if high browsing be so great an advantage, why
has not any other hoofed quadruped acquired a long neck and lofty stature,
besides the giraffe, and, in a lesser degree, the camel, guanaco and
macrauchenia? Or, again, why has not any member of the group acquired a
long proboscis? With respect to South Africa, which was formerly inhabited
by numerous herds of the giraffe, the answer is not difficult, and can best
be given by an illustration. In every meadow in England, in which trees
grow, we see the lower branches trimmed or planed to an exact level by the
browsing of the horses or cattle; and what advantage would it be, for
instance, to sheep, if kept there, to acquire slightly longer necks? In
every district some one kind of animal will almost certainly be able to
browse higher than the others; and it is almost equally certain that this
one kind alone could have its neck elongated for this purpose, through
natural selection and the effects of increased use. In South Africa the
competition for browsing on the higher branches of the acacias and other
trees must be between giraffe and giraffe, and not with the other ungulate

Why, in other quarters of the world, various animals belonging to this same
order have not acquired either an elongated neck or a proboscis, cannot be
distinctly answered; but it is as unreasonable to expect a distinct answer
to such a question as why some event in the history of mankind did not
occur in one country while it did in another. We are ignorant with respect
to the conditions which determine the numbers and range of each species,
and we cannot even conjecture what changes of structure would be favourable
to its increase in some new country. We can, however, see in a general
manner that various causes might have interfered with the development of a
long neck or proboscis. To reach the foliage at a considerable height
(without climbing, for which hoofed animals are singularly ill-constructed)
implies greatly increased bulk of body; and we know that some areas support
singularly few large quadrupeds, for instance South America, though it is
so luxuriant, while South Africa abounds with them to an unparalleled
degree. Why this should be so we do not know; nor why the later tertiary
periods should have been much more favourable for their existence than the
present time. Whatever the causes may have been, we can see that certain
districts and times would have been much more favourable than others for
the development of so large a quadruped as the giraffe.

In order that an animal should acquire some structure specially and largely
developed, it is almost indispensable that several other parts should be
modified and coadapted. Although every part of the body varies slightly,
it does not follow that the necessary parts should always vary in the right
direction and to the right degree. With the different species of our
domesticated animals we know that the parts vary in a different manner and
degree, and that some species are much more variable than others. Even if
the fitting variations did arise, it does not follow that natural selection
would be able to act on them and produce a structure which apparently would
be beneficial to the species. For instance, if the number of individuals
existing in a country is determined chiefly through destruction by beasts
of prey--by external or internal parasites, etc.--as seems often to be the
case, then natural selection will be able to do little, or will be greatly
retarded, in modifying any particular structure for obtaining food.
Lastly, natural selection is a slow process, and the same favourable
conditions must long endure in order that any marked effect should thus be
produced. Except by assigning such general and vague reasons, we cannot
explain why, in many quarters of the world, hoofed quadrupeds have not
acquired much elongated necks or other means for browsing on the higher
branches of trees.

Objections of the same nature as the foregoing have been advanced by many
writers. In each case various causes, besides the general ones just
indicated, have probably interfered with the acquisition through natural
selection of structures, which it is thought would be beneficial to certain
species. One writer asks, why has not the ostrich acquired the power of
flight? But a moment's reflection will show what an enormous supply of
food would be necessary to give to this bird of the desert force to move
its huge body through the air. Oceanic islands are inhabited by bats and
seals, but by no terrestrial mammals; yet as some of these bats are
peculiar species, they must have long inhabited their present homes.
Therefore Sir C. Lyell asks, and assigns certain reasons in answer, why
have not seals and bats given birth on such islands to forms fitted to live
on the land? But seals would necessarily be first converted into
terrestrial carnivorous animals of considerable size, and bats into
terrestrial insectivorous animals; for the former there would be no prey;
for the bats ground-insects would serve as food, but these would already be
largely preyed on by the reptiles or birds, which first colonise and abound
on most oceanic islands. Gradations of structure, with each stage
beneficial to a changing species, will be favoured only under certain
peculiar conditions. A strictly terrestrial animal, by occasionally
hunting for food in shallow water, then in streams or lakes, might at last
be converted into an animal so thoroughly aquatic as to brave the open
ocean. But seals would not find on oceanic islands the conditions
favourable to their gradual reconversion into a terrestrial form. Bats, as
formerly shown, probably acquired their wings by at first gliding through
the air from tree to tree, like the so-called flying squirrels, for the
sake of escaping from their enemies, or for avoiding falls; but when the
power of true flight had once been acquired, it would never be reconverted
back, at least for the above purposes, into the less efficient power of
gliding through the air. Bats, might, indeed, like many birds, have had
their wings greatly reduced in size, or completely lost, through disuse;
but in this case it would be necessary that they should first have acquired
the power of running quickly on the ground, by the aid of their hind legs
alone, so as to compete with birds or other ground animals; and for such a
change a bat seems singularly ill-fitted. These conjectural remarks have
been made merely to show that a transition of structure, with each step
beneficial, is a highly complex affair; and that there is nothing strange
in a transition not having occurred in any particular case.

Lastly, more than one writer has asked why have some animals had their
mental powers more highly developed than others, as such development would
be advantageous to all? Why have not apes acquired the intellectual powers
of man? Various causes could be assigned; but as they are conjectural, and
their relative probability cannot be weighed, it would be useless to give
them. A definite answer to the latter question ought not to be expected,
seeing that no one can solve the simpler problem, why, of two races of
savages, one has risen higher in the scale of civilisation than the other;
and this apparently implies increased brain power.

We will return to Mr. Mivart's other objections. Insects often resemble
for the sake of protection various objects, such as green or decayed
leaves, dead twigs, bits of lichen, flowers, spines, excrement of birds,
and living insects; but to this latter point I shall hereafter recur. The
resemblance is often wonderfully close, and is not confined to colour, but
extends to form, and even to the manner in which the insects hold
themselves. The caterpillars which project motionless like dead twigs from
the bushes on which they feed, offer an excellent instance of a resemblance
of this kind. The cases of the imitation of such objects as the excrement
of birds, are rare and exceptional. On this head, Mr. Mivart remarks, "As,
according to Mr. Darwin's theory, there is a constant tendency to
indefinite variation, and as the minute incipient variations will be in ALL
DIRECTIONS, they must tend to neutralize each other, and at first to form
such unstable modifications that it is difficult, if not impossible, to see
how such indefinite oscillations of infinitesimal beginnings can ever build
up a sufficiently appreciable resemblance to a leaf, bamboo, or other
object, for natural selection to seize upon and perpetuate."

But in all the foregoing cases the insects in their original state no doubt
presented some rude and accidental resemblance to an object commonly found
in the stations frequented by them. Nor is this at all improbable,
considering the almost infinite number of surrounding objects and the
diversity in form and colour of the hosts of insects which exist. As some
rude resemblance is necessary for the first start, we can understand how it
is that the larger and higher animals do not (with the exception, as far as
I know, of one fish) resemble for the sake of protection special objects,
but only the surface which commonly surrounds them, and this chiefly in
colour. Assuming that an insect originally happened to resemble in some
degree a dead twig or a decayed leaf, and that it varied slightly in many
ways, then all the variations which rendered the insect at all more like
any such object, and thus favoured its escape, would be preserved, while
other variations would be neglected and ultimately lost; or, if they
rendered the insect at all less like the imitated object, they would be
eliminated. There would indeed be force in Mr. Mivart's objection, if we
were to attempt to account for the above resemblances, independently of
natural selection, through mere fluctuating variability; but as the case
stands there is none.

Nor can I see any force in Mr. Mivart's difficulty with respect to "the
last touches of perfection in the mimicry;" as in the case given by Mr.
Wallace, of a walking-stick insect (Ceroxylus laceratus), which resembles
"a stick grown over by a creeping moss or jungermannia." So close was this
resemblance, that a native Dyak maintained that the foliaceous excrescences
were really moss. Insects are preyed on by birds and other enemies whose
sight is probably sharper than ours, and every grade in resemblance which
aided an insect to escape notice or detection, would tend towards its
preservation; and the more perfect the resemblance so much the better for
the insect. Considering the nature of the differences between the species
in the group which includes the above Ceroxylus, there is nothing
improbable in this insect having varied in the irregularities on its
surface, and in these having become more or less green-coloured; for in
every group the characters which differ in the several species are the most
apt to vary, while the generic characters, or those common to all the
species, are the most constant.

The Greenland whale is one of the most wonderful animals in the world, and
the baleen, or whalebone, one of its greatest peculiarities. The baleen
consists of a row, on each side of the upper jaw, of about 300 plates or
laminae, which stand close together transversely to the longer axis of the
mouth. Within the main row there are some subsidiary rows. The
extremities and inner margins of all the plates are frayed into stiff
bristles, which clothe the whole gigantic palate, and serve to strain or
sift the water, and thus to secure the minute prey on which these great
animals subsist. The middle and longest lamina in the Greenland whale is
ten, twelve, or even fifteen feet in length; but in the different species
of Cetaceans there are gradations in length; the middle lamina being in one
species, according to Scoresby, four feet, in another three, in another
eighteen inches, and in the Balaenoptera rostrata only about nine inches in
length. The quality of the whalebone also differs in the different

With respect to the baleen, Mr. Mivart remarks that if it "had once
attained such a size and development as to be at all useful, then its
preservation and augmentation within serviceable limits would be promoted
by natural selection alone. But how to obtain the beginning of such useful
development?" In answer, it may be asked, why should not the early
progenitors of the whales with baleen have possessed a mouth constructed
something like the lamellated beak of a duck? Ducks, like whales, subsist
by sifting the mud and water; and the family has sometimes been called
Criblatores, or sifters. I hope that I may not be misconstrued into saying
that the progenitors of whales did actually possess mouths lamellated like
the beak of a duck. I wish only to show that this is not incredible, and
that the immense plates of baleen in the Greenland whale might have been
developed from such lamellae by finely graduated steps, each of service to
its possessor.

The beak of a shoveller-duck (Spatula clypeata) is a more beautiful and
complex structure than the mouth of a whale. The upper mandible is
furnished on each side (in the specimen examined by me) with a row or comb
formed of 188 thin, elastic lamellae, obliquely bevelled so as to be
pointed, and placed transversely to the longer axis of the mouth. They
arise from the palate, and are attached by flexible membrane to the sides
of the mandible. Those standing towards the middle are the longest, being
about one-third of an inch in length, and they project fourteen one-
hundredths of an inch beneath the edge. At their bases there is a short
subsidiary row of obliquely transverse lamellae. In these several respects
they resemble the plates of baleen in the mouth of a whale. But towards
the extremity of the beak they differ much, as they project inward, instead
of straight downward. The entire head of the shoveller, though
incomparably less bulky, is about one-eighteenth of the length of the head
of a moderately large Balaenoptera rostrata, in which species the baleen is
only nine inches long; so that if we were to make the head of the shoveller
as long as that of the Balaenoptera, the lamellae would be six inches in
length, that is, two-thirds of the length of the baleen in this species of
whale. The lower mandible of the shoveller-duck is furnished with lamellae
of equal length with these above, but finer; and in being thus furnished it
differs conspicuously from the lower jaw of a whale, which is destitute of
baleen. On the other hand, the extremities of these lower lamellae are
frayed into fine bristly points, so that they thus curiously resemble the
plates of baleen. In the genus Prion, a member of the distinct family of
the Petrels, the upper mandible alone is furnished with lamellae, which are
well developed and project beneath the margin; so that the beak of this
bird resembles in this respect the mouth of a whale.

>From the highly developed structure of the shoveller's beak we may proceed
(as I have learned from information and specimens sent to me by Mr.

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